The invention is based on a fuel injection device having a fuel pump a pump work chamber and an injection line connected to the pump work chamber. The injection line communicates with an injection nozzle via an interposed valve closing toward the pump work chamber. At the injection end and visa a control valve connected to the pump work chamber, fuel can be supplied under pressure to the side remote from the pump work chamber of the valve that closes toward the work chamber, and also via this valve, a connection to the return line can be opened up.
A device of this type is found for instance in European Patent Document Al-204 982. This fuel injection device is a so-called unit fuel injector, in which the pump piston is axially displaceable coaxially with the injection nozzle in a unit fuel injector housing. In the embodiment of Al-204 982, the shutoff of the injection event is effected in the conventional manner via control grooves on the circumference of the pump piston; via a groove provided on the pump piston, the pressure can rapidly be decreased as the piston overtakes a diversion line connected to the pump cylinder. The valve interposed between the pump work chamber and the injection nozzle is embodied as a pressure valve and is opened by the pressure in the pump work chamber, once an opening pressure predetermined by the spring of the pressure valve, whereupon the path of the fuel to the injection nozzle is opened up. In this embodiment, once the relief bore in the cylinder of the pump has been overtaken, fuel under pressure is forced into the spring chamber of a nozzle needle, which causes the closure of the nozzle needle in the direction of the spring that loads the nozzle needle. At the same time, such fuel, expelled via the diversion bore, is removed to a low-pressure line, in particular a return flow line; as a result, because of the resultant pressure drop in the pump work chamber, the pressure valve in the line to the nozzle can be closed by the force of the spring. In this known embodiment, the proposal is also made that some of this fuel diverted via the overflow line be carried into the spring chamber of the pressure valve in the line to the injection nozzle, to facilitate closure of the pressure valve. However, the actual closing stroke of the pressure valve in the known embodiment is substantially determined by the dimensions of the spring of this pressure valve, and the adaptation of this spring must be selected taking the spring characteristic of the nozzle needle spring into account, in order to assure a satisfactory function.
In fuel injection devices of the above type, especially in unit fuel injectors, it has already been proposed that instead of the diversion via control grooves of the pump piston, magnetic valves be used, to enable more precis adjustment of the instant of the end of injection by abruptly dropping the pressure.